#include <stdio.h>

void chmin(int* a, int b)
{
	if (*a > b) *a = b;
}

int main()
{
	int i, N, A[5001];
	scanf("%d", &N);
	for (i = 1; i <= N; i++) scanf("%d", &(A[i]));

	const int sup = 1 << 28;
	int j, k, cur, prev, _dp[2][10001], *dp[2] = {&(_dp[0][5000]), &(_dp[1][5000])}, num[2];
	for (i = -N; i <= N; i++) dp[0][i] = sup;
	for (i = 1, cur = 1, prev = 0, dp[0][0] = 0; i <= N; i = j + 1, cur ^= 1, prev ^= 1) {
		for (j = i, num[0] = 0, num[1] = 0; j <= N && A[j] != 2; j++) num[A[j]]++;
		for (k = -N; k <= N; k++) dp[cur][k] = sup;
		for (k = -N; k <= N; k++) {
			if (dp[prev][k] == sup) continue;
			chmin(&(dp[cur][k+num[0]]), dp[prev][k] + num[0]);
			chmin(&(dp[cur][k-num[1]]), dp[prev][k] + num[1]);
		}
	}
	if (dp[prev][0] < sup) printf("%d\n", dp[prev][0] / 2);
	else printf("-1\n");
	fflush(stdout);
	return 0;
}